The present invention relates to producing a colour picture tube screen, particularly a high definition screen for use in a Datagraphic Display (DGD) colour picture tube.
A difference between a normal colour television display tube screen and a high definition screen for DGD tubes is that the normal colour television screen comprises triplets of phosphor stripes which luminesce in different colours whereas for a DGD tube the screen normally comprises phosphor dots disposed in apertures of a black light absorbing matrix. The making of screens for both types of tubes involves exposing a photoresist material applied to the internal surface of a faceplate panel to light from a point light source which is projected onto the faceplate panel by means of a lens. The lens is designed so that the angle at which the light impinges on the photoresist corresponds to the trajectory of an electron beam to that point on the screen. In the case of making a colour television screen the lens is a continuous lens whereas a segmented lens comprising a plurality of rectilinearly arranged contiguous facets having slightly different inclinations with respect to each other is frequently used for making high resolution DGD tube screens.
British Patent Specification 1473388 discloses a method of screening a colour television picture tube by exposing a photosensitive material on a support to light emitted from a light source and passed through a segmented lens having a plurality of inclined facets, the junctions between adjacent facets being formed by discontinuous surfaces. In order to avoid an objectionable image pattern being produced due to light scattering at the discontinuous surfaces of the segmented lens, these discontinuous surfaces are masked and the masked lens is reciprocated (or wobbled) in an oblique linear direction of 45 annular degrees to the two orthogonal directions of the discontinuities during exposure of the photosensitive material on the faceplate. Typically the extent of the motion is equivalent to the distance between the centres of two diagonally adjacent lens elements and back. A drawback of such a technique is that unless the inclination of all the facets is the same, the energy distribution on the material applied to the faceplate will not be equal. Consequently the unequal energy distribution manifests itself as light areas interspersed by narrow dark and bright lines in those places where the facet images are separated from each other or partially overlap each other, respectively.